Arc flash detection apparatus and electrical system including the same

ABSTRACT

An electrical system includes first, second and third busses; a first interrupter electrically connected between the first and second busses; at least one of a shorting apparatus operatively associated with the first or second bus, and the first interrupter comprising a trip coil; a current sensor to sense a fault current flowing in the first bus and responsively output a first signal; a number of light sensors to sense an arc flash operatively associated with a number of the first, second or third busses and responsively output a second signal; a second interrupter electrically connected between the second and third power busses and output a third signal; and a circuit to invert the third signal to provide a fourth signal, and to operate the at least one of the shorting apparatus and the trip coil responsive to an AND of the first, second and fourth signals.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of, and claims priority under 35U.S.C. § 120 from, U.S. patent application Ser. No. 15/668,382, filedAug. 3, 2017, entitled “ARC FLASH DETECTION APPARATUS AND ELECTRICALSYSTEM INCLUDING THE SAME”, the contents of which are incorporatedherein by reference.

BACKGROUND Field

The disclosed concept pertains generally to electrical systems and, moreparticularly, to electrical power systems that are subject to arcflashes. The disclosed concept also pertains to arc flash detectionapparatus.

Background Information

Electric power systems incorporate switches for control and protectionpurposes. Distribution systems, which form part of the overall electricpower system, include main and feeder power buses and circuit breakersmounted in metal cabinets to form switchgear. Interruption of currentflow in the buses of the distribution system by a circuit breakercreates an arc as the contacts of the circuit breaker open. These arcscaused by interruption are generally contained and extinguished in thenormal course of operation of the circuit breaker.

At times, however, unintended arcing faults can occur within switchgearcabinets, such as between power buses, or between a power bus and agrounded metal component. Such arcing faults can produce high energygases, which pose a threat to the structure and nearby personnel. Thisis especially true when maintenance is performed on or about live powercircuits. For example, a worker might inadvertently short out the powerbus, thereby creating an arcing fault inside the enclosure. Theresulting arc blast creates an extreme hazard and could cause injury oreven death. This problem is exacerbated by the fact that the enclosuredoors are typically open for maintenance.

A common approach to protecting personnel from arcing faults inswitchgear has been to design the metal enclosures to withstand theblast from the arcing fault. This has been done at great additionalcosts due to the heavy gauge metal used and numerous weld joints neededto prevent flying debris. Even with these precautions, the blast from anarcing fault inside the switchgear may not be contained.

Various known methods seek to minimize the severity of the blast from aninternal arcing fault. These methods include pressure sensing and lightdetection, which sense the arcing fault within the switchgear and causea circuit breaker to trip before significant damage can result. Thepressure sensing method is limited by the insensitivity of the pressuresensors. By the time cabinet pressure has risen to detectable levels,the arcing fault has already caused significant damage.

In an electrical system, an internal arcing fault can occur somewhereinside of the switchgear enclosure, frequently, but certainly notlimited to the point where the power cables servicing the load areconnected.

In an electrical system, such as, for example, a motor control center,an internal arcing fault could occur within the load center panelboardwhen, for example, servicing line panelboards. A bare live copper buscould inadvertently be shorted. Another example for both low and mediumvoltage systems would be the shorting of power conductors by rodents,snakes, or other animals or objects.

In the low voltage system, the arcing fault could clear itself, byburning or ejecting the short, but it may take more than one-half cycleto do so, thereby causing significant damage and great risk of injury toworkers even in one-half cycle of arcing.

A medium voltage system could behave similar to a low voltage system;however, the medium voltage system would be less likely to beself-extinguishing.

It is known to employ a high-speed shorting switch to eliminate anarcing fault. Known arc elimination devices and systems produce a boltedfault across the power bus (e.g., phase-to-phase, such as two switchesfor three phases; phase-to-ground, such as three switches for threephases), in order to eliminate the arcing fault and prevent equipmentdamage and personnel injury due to arc blasts. It is also known toemploy various types of crowbar switches for this purpose. The resultingshort on the power bus causes an upstream circuit breaker to clear thebolted fault by removing power. See, for example, U.S. Pat. Nos.7,145,757; 7,035,068; 6,839,209; 6,724,604; 6,693,438; 6,657,150; and6,633,009. As a result, system power is lost due to the tripping of theupstream circuit breaker. Once the arc is out, and if the short has beenburned away or removed, then system power can be restored.

Arc flash light detection systems can employ only the light produced byarcing internal to electrical equipment (see, for example, U.S. Pat. No.6,229,680), or can sense a combination of light and relatively highcurrent. The addition of current sensing is intended to avoid nuisanceoperation for normal light sources (e.g., a camera flash; a flashlight).Protective devices, such as air circuit breakers (i.e., circuit breakersthat interrupt current in air), produce arc bi-products during normaloperation, such as, for example, copper vapor in the arc plasmaexhausted from a circuit breaker's arc chute. Since such protectivedevices also operate during relatively high current conditions, thenormal operation of these protective devices with an open arc chamberproduces challenges when attempting to protect such devices against thecondition of internal arcing, yet also make them immune to the normalarcing such devices produce during relatively high current protectionconditions. U.S. Pat. No. 8,228,652 describes an approach which utilizesa delay circuit to try to avoid nuisance operation due to arcs emanatingfrom an open circuit interrupter. While such approach addresses somenuisance operation, there is still room for improvement in electricalsystems for differentiating between arcs due to an internal fault versusarcs emanating from an open circuit interrupter.

There is also room for improvement in arc flash detection apparatus.

SUMMARY

These needs and others are met by embodiments of the disclosed concept,which are directed to an arc flash detection apparatus and an electricalsystem including an arc flash detection apparatus.

As one aspect of the disclosed concept, an electrical system isprovided. The electrical system comprises: a first power bus; a secondpower bus; a third power bus; a first circuit interrupter electricallyconnected between the first power bus and the second power bus; at leastone of: (a) a shorting apparatus operatively associated with the firstpower bus or second power bus, and (b) the first circuit interruptercomprising a trip coil; a current sensor structured to sense a faultcurrent of at least a predetermined magnitude flowing in the first powerbus and responsively output a first logical signal; a number of lightsensors structured to sense an arc flash operatively associated with anumber of the first power bus, second power bus and the third power busand responsively output a second logical signal; a second circuitinterrupter electrically connected between the second power bus and thethird power bus, the second circuit interrupter being structured to movefrom a closed position to an open position responsive to detecting anovercurrent condition and responsively output a third logical signal;and a circuit structured invert the third logical signal to provide afourth logical signal, and to operate the at least one of the shortingapparatus and the trip coil responsive to a logical AND of the firstlogical signal, the second logical signal and the fourth logical signal.

The current sensor may be structured to sense the fault current andoutput the first logical signal when the sensed fault current exceeds apredetermined magnitude.

The second circuit interrupter may comprise a protective relay and thethird logical signal may originate from the protective relay.

The second circuit interrupter may comprise an auxiliary contact and thethird logical signal may originate from the auxiliary contact.

The second circuit interrupter may comprise a trip shaft and the thirdlogical signal may be produced by a device monitoring the trip shaft.

The circuit may comprise a three-input AND gate structured to providethe logical AND of the first logical signal, the second logical signaland the fourth logical signal, and an output to operate the at least oneof the shorting apparatus and the trip coil.

The electrical system may be disposed within a housing; wherein thethird power bus extends out of the housing; and wherein the circuit isstructured to operate the shorting apparatus for a fault on the secondpower bus and for a fault on the third power bus occurring within thehousing but is structured to not operate the shorting apparatus for afault on the third power bus occurring outside of the housing or for anarc generated by the second current interrupter when protecting againstthe fault on the third power bus occurring outside of the housing.

The circuit may comprise an arc detection relay.

As another aspect of the disclosed concept, an arc flash detectionapparatus for an electrical system comprises a first power bus, a secondpower bus, a third power bus, a first circuit interrupter electricallyconnected between the first power bus and the second power bus, a secondcircuit interrupter electrically connected between the second power busand the third power bus, the second circuit interrupter being structuredto move from a closed position to an open position responsive todetecting an overcurrent condition and responsively output a thirdlogical signal, and at least one of: (a) a shorting apparatusoperatively associated with the second power bus, and (b) the firstcircuit interrupter comprising a trip coil, is provided. The arc flashdetection apparatus comprises: a current sensor structured to sense afault current of at least a predetermined magnitude flowing in thesecond power bus and responsively output a first logical signal; anumber of light sensors structured to sense an arc flash operativelyassociated with a number of the second power bus and the third power busand responsively output a second logical signal; a second circuitinterrupter electrically connected between the second power bus and thethird power bus, the second circuit interrupter being structured to movefrom a closed position to an open position responsive to detecting anovercurrent condition and responsively output a third logical signal;and a circuit structured to invert the third logical signal to provide afourth logical signal, and to operate the at least one of the shortingapparatus and the trip coil responsive to a logical AND of the firstlogical signal, the second logical signal and the fourth logical signal.

The current sensor may be structured to output the first logical signalwhen the sensed fault current exceeds a predetermined magnitude.

The circuit may comprise a three-input AND gate structured to providethe logical AND of the first logical signal, the second logical signaland the fourth logical signal, and an output to operate the at least oneof the shorting apparatus and the trip coil.

BRIEF DESCRIPTION OF THE DRAWINGS

A full understanding of the disclosed concept can be gained from thefollowing description of the preferred embodiments when read inconjunction with the accompanying drawings in which:

FIG. 1 is a block diagram in schematic form of an arc flash detectionapparatus for use with switchgear comprising source service, main andfeeder load side power buses, main and feeder circuit breakers, and ashorting device on the main power bus, with a fault outside of theequipment housing on the feeder load side power bus in accordance withan embodiment of the disclosed concept.

FIG. 2 includes plots of various signals versus time for the arc flashdetection apparatus of FIG. 1.

FIG. 3 is a block diagram in schematic form of the arc flash detectionapparatus of FIG. 1, except with a fault inside of the equipment housingon the feeder load side power bus in accordance with an embodiment ofthe disclosed concept.

FIG. 4 includes plots of various signals versus time for the arc flashdetection apparatus of FIG. 3.

FIG. 5 is a block diagram in schematic form of an arc flash detectionapparatus for use with switchgear comprising source service, main andfeeder load side power buses, main and feeder circuit breakers, withouta shorting device on the main power bus, with a fault inside of theequipment housing on the feeder load side power bus in accordance withanother embodiment of the disclosed concept.

FIG. 6 includes plots of various signals versus time for the arc flashdetection apparatus of FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As employed herein, the term “number” shall mean one or an integergreater than one (i.e., a plurality).

As employed herein, the term “processor” means a programmable analogand/or digital device that can store, retrieve, and process data; acomputer; a workstation; a personal computer; a microprocessor; amicrocontroller; a microcomputer; a central processing unit; a mainframecomputer; a mini-computer; a server; a networked processor; or anysuitable processing device or apparatus.

Referring to FIG. 1, an electrical system 2 in accordance with anexample embodiment of the disclosed concept includes a first power bus 4(e.g., without limitation, a source service power bus), a second powerbus 6 (e.g., without limitation, a main power bus) and a third power bus8 (e.g., without limitation, a feeder load side power bus). A firstcircuit interrupter 10 (e.g., without limitation, a main circuitbreaker) is electrically connected between the first and second powerbusses 4,6 such that the second power bus 6 is electrically isolatedfrom the first power bus 4 when the first circuit interrupter 10 isdisposed in an “open” position (conversely, the first and second powerbusses 4,6 are electrically connected when the first circuit interrupter10 is disposed in a “closed” position). A second circuit interrupter 12(e.g., without limitation, a feeder circuit breaker) provided in ahousing 13 is electrically connected between the second and third powerbusses 6,8 such that the third power bus 8 is electrically isolated fromthe second power bus 6 when the second circuit interrupter 12 isdisposed in an “open” position (conversely, the second and third powerbusses 6,8 are electrically connected when the second circuitinterrupter 12 is disposed in a “closed” position). Although both areshown in the example of FIG. 1, the electrical system 2 can include oneor both of a shorting apparatus, such as shorting device 14, operativelyassociated with the second power bus 6, and a trip coil, such as a shunttrip coil (not shown) of the first circuit interrupter 10. It will beappreciated that the first circuit interrupter 10 can be part of anelectrical enclosure 16 for the second power bus 6 and the secondcircuit interrupter 12, or can be part of a separate assembly (notshown).

The example electrical system 2 further includes a current sensor 18structured to sense a fault current 19 at the incoming of electricalenclosure 16 of at least a predetermined magnitude flowing in the secondpower bus 6 and responsively output a first logical signal 20. A numberof light sensors 22 (two example light sensors 22 are shown, althoughany suitable quantity can be employed) are structured to sense an arcflash (e.g., arc flash 24 of FIG. 1) operatively associated with anumber of the second power bus 6 and the third power bus 8 andresponsively output a second logical signal 26.

Second circuit interrupter 12 is a circuit interrupter that isstructured to interrupt the flow of current therethrough in air.Suitable examples of mechanisms which may be employed as second circuitinterrupter 12 include, without limitation, power circuit breakers,molded case circuit breakers, insulated case circuit breakers, loadbreaking switches. In contrast, circuit interrupters that that arestructured to interrupt the flow of current therethough in a vacuum oroil are not suitable for use as second interrupter 12. Second circuitinterrupter 12 is structured to produce a third logic signal 28, alsoreferred to herein as a blocking signal or blocking input, wheneversecond circuit interrupter is about to clear a fault. Such logic signal28 may originate from one or more of a plurality of sources, e.g.,without limitation: a protective relay of second circuit interrupter 12;an auxiliary contact of second circuit interrupter 12; any devicemonitoring the trip shaft of second circuit interrupter 12; the tripactuator of second circuit interrupter 12; or any similar device.

A circuit 30 is structured to invert the third logical signal 28 toprovide a fourth logical signal 32, and to operate at least one of theshorting device 14 and the first circuit interrupter 10 responsive to alogical AND, such as is provided by an example three-input AND gate 33,of the first logical signal 20, the second logical signal 26 and thefourth logical signal 32. The three-input AND gate 33 has an output 34to operate at least one of the shorting device 14 and the first circuitinterrupter 10 or a plurality of circuit interrupters outside ofenclosure 16.

The example third power bus 8 can comprise any, some or all of a numberof power busses (not shown), a number of power conductors (not shown), anumber of power cables (not shown), and/or a number of loads (notshown), such as equipment (not shown) electrically connected external toenclosure 13 housing the second circuit interrupter 12 on the “thirdpower bus side” (e.g., to the right with respect to FIG. 1) of thesecond circuit interrupter 12.

The example current sensor 18 (e.g., without limitation, a currenttransformer (CT); current sensor, a Rogowski coil; a Rogowski sensor) isstructured to sense the fault current 19 and output the first logicalsignal 20 when the sensed fault current exceeds a predeterminedmagnitude. For example and without limitation, a current threshold ofabout two times the nominal CT rating can be employed. For example, thisensures that light sensing does not activate the shorting device 14and/or the first circuit interrupter 10 due to normal or rated loadcurrent. Alternatively, any suitable current threshold can be employed.

In FIG. 2, the first logical (current) signal 20 is output by thecurrent sensor 18, which senses primary current flow, such as the faultcurrent 19 being of at least the predetermined magnitude flowing in thesecond power bus 6. In the case of an internal fault, such as fault 40shown in FIG. 3, the resulting light 42 and fault current 19 occuressentially simultaneously. Conversely, for an external fault, such asfault 44 shown in FIG. 1, fault current 19 flows for a relatively longperiod of time, as can be seen between the leading edges of the signals20 and 26 of FIG. 2, prior to the arc flash 24 from arc chutes (notshown) being generated from interruption of the fault current 19 by thesecond circuit interrupter 12.

The disclosed concept need not operate a circuit interrupter, such asthe first circuit interrupter 10, and can advantageously prevent thenuisance operation thereof, since the second circuit interrupter 12 ispermitted to interrupt the external fault 44 (FIG. 1), as shown in FIG.2, without operation of the shorting device 14 that would otherwisecause the first circuit interrupter 10 to open. As shown in FIGS. 1 and2, the second circuit interrupter 12 trips open, and in doing so,produces the arc flash 24 under normal operating conditions withoutoperating the shorting device 14.

Conversely, as shown in FIGS. 3 and 4, an internal fault 40 (andassociated arc flash 42) causes operation of the shorting device 14that, in turn, causes the first circuit interrupter 10 to open (andsubsequent arc flash 45 to occur).

Alternatively, the disclosed concept need not employ or operate theshorting device 14. Here, when output 34 of the three-input AND gate 32is true, this causes a contact (not shown) to close, actuate theshorting device 14 and, thus, trip open the first circuit interrupter10. As has been discussed, each of the shorting device 14, which isactuated by the three-input AND gate output 34, and the first circuitinterrupter 10 can be separately employed or can be employed together incombination.

The example circuit 30 can be any suitable analog and/or digitalcircuit, such as a hardware circuit and/or a processor-based (e.g.,hardware and software/firmware) circuit. For example and withoutlimitation, this could be a combination of digital and analog technologywith embedded firmware. In an example embodiment, circuit 30 is an arcfault relay.

As can be seen from FIGS. 4 and 2, the circuit 30 can operate theshorting device 14 for the internal fault 40 (and resulting arc flash 42in FIG. 3) on the second power bus 6, but it does not operate theshorting device 14 for the external fault 44 (FIG. 1) on the third powerbus 8 or for the arc flash 24 from arc chutes (not shown) beinggenerated from interruption of the fault current 19 by the secondcircuit interrupter 12 when protecting against such external fault 44.The circuit 30, the current sensor 18 and the number of light sensors 22provide an arc flash detection apparatus 50 for the electrical system 2.

FIG. 2 shows the current signal 20 output by the current sensor 18, aninternal trip signal 52 of the second circuit interrupter 12, andblocking input signal 28. The breaker interrupt signal 43 (which occursmechanically within the breaker and thus is not shown in FIG. 1) showsthe timing of the interruption of the fault current 19 by the secondcircuit interrupter 12. The signal 26 shows the timing of the sensing ofthe arc flash 24 from the arc chutes (not shown) of the secondinterrupter 12. The arc flash 24 is generated from interruption of thefault current 19 by the second circuit interrupter 12. Signals 54 and 56show that there is no signal to the shorting device 14 (or the firstcircuit interrupter 10), and that there is no operation of the same,since the output of three-input AND gate 33 is always false (sincesignal 32 is false when signal 28 is true, i.e., when second circuitinterrupter 12 is moving to an open position).

FIG. 4 shows that there is no internal trip signal 52, no circuitbreaker interrupt signal and no interruption of the fault current 19 bythe second circuit interrupter 12 as fault interruption happens at firstcircuit interrupter 10, since there is only the internal fault 40 (andresulting arc flash 42 in FIG. 3). Here, unlike FIG. 2, the signal 26follows the current signal 20, since there is the internal fault 40 (andresulting arc flash 42 in FIG. 3). Signal 34 shows that there is thesignal 54 to the shorting device 14, since the output 34 of three-inputAND gate 33 is true when the three input signals, i.e., signals 26, 32(i.e., signal 28 is false thus 32 is true) and 20 are true.

Referring to FIG. 5, an electrical system 2′ is similar to theelectrical system 2 of FIG. 1, except that there is no shorting device,such as shorting device 14 of electrical system 2 shown in FIGS. 1 and3. FIG. 6 is similar to FIG. 4, except that there is no operation of ashorting device.

The disclosed concept can be employed in any electrical system that hasan upstream circuit interrupter that can open when a local or internalarc flash event occurs. Some non-limiting applications of electricalsystems include low voltage or medium voltage switchgear, motor controland switchboards.

While specific embodiments of the disclosed concept have been describedin detail, it will be appreciated by those skilled in the art thatvarious modifications and alternatives to those details could bedeveloped in light of the overall teachings of the disclosure.Accordingly, the particular arrangements disclosed are meant to beillustrative only and not limiting as to the scope of the disclosedconcept which is to be given the full breadth of the claims appended andany and all equivalents thereof

What is claimed is:
 1. An electrical system comprising: a first powerbus; a second power bus; a third power bus; a first circuit interrupterelectrically connected between the first power bus and the second powerbus; a shorting apparatus operatively associated with the first powerbus or second power bus; a current sensor structured to sense a faultcurrent of at least a predetermined magnitude flowing in the first powerbus and responsively output a first signal; a number of light sensorsstructured to sense an arc flash operatively associated with a number ofthe first power bus, second power bus and the third power bus andresponsively output a second signal; a second circuit interrupterelectrically connected between the second power bus and the third powerbus, the second circuit interrupter being structured to move contactsfrom a closed position to an open position responsive to detecting anovercurrent condition and responsively output a third signal when thesecond circuit interrupter is about to clear a fault; and a circuitstructured to receive the first signal, the second signal, and the thirdsignal and to selectively activate or not activate the shortingapparatus dependent on receiving at least one of the signals.
 2. Theelectrical system of claim 1, wherein the circuit is configured toactivate the at least one of the shorting device or the first circuitinterrupter responsive to receiving the first signal and the secondsignal without having first received the third signal.
 3. The electricalsystem of claim 1, wherein the circuit is configured to activate theshorting apparatus responsive to receiving the second signal withouthaving first received the third signal.
 4. A circuit for use in anelectrical system having at least one of a shorting device or a firstcircuit interrupter, a second circuit interrupter, and an enclosure, thecircuit being configured to: receive a first signal indicative of thepresence of a fault current in the electrical system; receive a secondsignal indicative of the presence of light in the enclosure; receive ablocking signal indicative of commencement of movement of the secondcircuit interrupter from a closed position toward an open position whenthe second circuit interrupter is about to clear the fault; andselectively activate or not activate the at least one of the shortingdevice or the first circuit interrupter dependent on receiving at leastone of the signals.
 5. The circuit of claim 4, wherein the circuit isconfigured to activate the at least one of the shorting device or thefirst circuit interrupter responsive to receiving the first signal andthe second signal without having first received the blocking signal. 6.The circuit of claim 4, wherein the circuit is configured to activatethe at least one of the shorting device or the first circuit interrupterresponsive to receiving the second signal without having first receivedthe blocking signal.
 7. The circuit of claim 4, further comprising anauxiliary contact associated with the second circuit interrupter,wherein the blocking signal originates from the auxiliary contact. 8.The circuit of claim 4, further comprising a device monitoring a tripshaft or a trip actuator of the second circuit interrupter, wherein theblocking signal originates from the device.
 9. The circuit of claim 4,wherein, upon receipt of the blocking signal, the circuit is configuredto prevent activation of the at least one of the shorting device or thefirst circuit interrupter.
 10. A method of operating an arc flashdetection system, comprising: receiving a blocking signal from a circuitinterrupter indicative of movement of the circuit interrupter toward anopen position, and preventing operation of a shorting device in responseto receiving the blocking signal.
 11. The method of claim 10, furthercomprising the step of operating the shorting device in response to:receipt of a signal indicative of an overcurrent condition in anelectrical system, receipt of a signal indicative of the presence oflight in an enclosure of the electrical system, and the failure toreceive the blocking signal.
 12. The method of claim 10, wherein theblocking signal originates from an auxiliary contact associated with thecircuit interrupter.
 13. The method of claim 10, wherein the blockingsignal originates from a device monitoring movement of the trip shaft ora trip actuator of the circuit interrupter.
 14. The method of claim 10,further comprising the step of preventing operation of an upstreamcircuit interrupter in response to the blocking signal.